1. Field of the Invention
This invention relates to an optical type encoder, and particularly to an optical type encoder used, for example, in a lens barrel or the like having a magnification changing system for moving a lens in the direction of the optical axis thereof to thereby vary the focal length thereof and suitable for detecting, for example, the position of the lens relative to another lens or a camera body.
2. Related Background Art
Heretofore, in a lens barrel having a magnification changing system for moving a lens in the direction of the optical axis thereof to thereby vary the focal length thereof, as a method of controlling the amount of movement of the lens and the position of the lens relative to a camera body there has been, for example, a method comprising causing a cam ring formed with a cam slot and a guide pin mounted on a lens frame to cooperate with each other to rotate the cam ring, thereby driving a lens unit and controlling the relative position and the amount of movement of the lens unit.
Also, as an optical type encoder for detecting the position of such a lens, there has been proposed by the applicant one which is comprised of a slit plate provided with a plurality of slits and a photocoupler comprising light projecting and receiving elements for projecting light onto the slits and receiving slit light passed through the slits. The encoder is designed to be capable of detecting the relative movement of the slit plate and the photocoupler resulting from the movement of the lens to thereby detect the position of the lens.
In the optical type encoder described above, if the number of positions of the slits is increased, the difference between one slit and another slit, i.e., the manufacturing error between the slits in respective positions, becomes severe. This means that severity is required not only of the error between the slits, but also of the accuracy of the inclination of a guide portion guiding the lens and the accuracy of inclination of a sensor.